A BBC Wales survey has found a number of councils now leave the decision on extra servings to schools or caterers.
Powys bans pudding as seconds, Cardiff schools are urged to offer only extra bread, and Ceredigion extras are small, bringing complaints from hungry pupils.
But the Welsh government says it would never want children left hungry, and it will issue new guidance next year.School meal policy varies in many areas of Wales, and here are some examples.
Several areas, such as Bridgend, Neath Port Talbot, Pembrokeshire, Torfaen, Vale of Glamorgan and Wrexham have no set policy on seconds, and leave the decision up to schools.
Most counties say all primary pupils get the same size portions, but some, Anglesey, Denbighshire and Ceredigion vary servings.
But Powys says primary pupils all pay the same and cooks cannot give “significantly bigger portions” to older ones. But they do get slightly more potatoes and vegetables.
BBC Wales asked education authorities about school meals, and 17 of the 22 responded. But while all said children are still allowed to ask for seconds, they may get a different response depending on where they live.
Powys Council says its cooks must use their “discretion” when offering extra food, and while its schools are allowed to serve any leftover vegetables and bread, puddings as seconds are off the menu. Powys also says catering managers advise cooks and rely on their “discretion and understanding.”
But some pupils and parents have complained about these restrictions, which follow the adoption of the Welsh government’s Appetite for Life programme, which aims to raise nutritional standards and help tackle childhood obesity.
There have also been complaints about the size of portions for primary school pupils. They have protested that 10-11-year-olds will need more food than a four-year-old.
Figures from earlier this year show more than 28% of five-year-olds in Wales are overweight, with 12.5% of children classed as obese. Wales has a bigger problem than either England or Scotland.
The standards have been in force in primary schools since September 2012, and were introduced across secondary schools at the start of this term.
When the Appetite for Life plan was launched in 2008 then Education Minister Jane Hutt said a balanced diet was essential for the young to become healthy adults.
It stipulates, for instance, that at least two portions of fruit and vegetables must be available each day, and that chips cannot be served more than twice a week.
Oily fish must be served at least twice a month, but food cooked in fat or oil cannot be given to pupils more than twice a week.
Schools are told that bread should be on offer, but best eaten without spread, salt must not be available, and any sauces like tomato ketchup, salad cream and mayonnaise must only be in 10ml portions.
Cakes and biscuits can only be served as part of lunch, and cannot be served at other times in the school day.
Any meals made from mechanically recovered meat cannot be served and fresh drinking water should be freely available.
Primary pupils should receive 530 calories per two-course meal, while children in secondary schools get 646 calories.
Education Minister Huw Lewis has echoed those words and wants “a large dose of common sense” to be used in interpreting the guidelines.
He told AMs last month the guidelines are flexible and “offer a proper nutritional balance for our young people, and that is something that we have been working towards for a long time, and now we have it. So, the guidelines matter but so does common sense”.
In response to BBC Wales’ findings, a Welsh government spokesperson said:
“We would never want to see children having school meals going hungry.
“We have provided schools with suggested portion sizes which cater for changing nutritional requirements as children get older. So, for example, a child in year 6 would have a larger portion than a child in reception.”
“Local authorities and many schools have worked hard over the years to improve the quality of food and drink provided in schools, in line with the Appetite for Life recommended standards.
“However, in the absence of legislation, there has been a variable rollout across schools. As a consequence, not all schools were achieving the recommended standards. The Healthy Eating in Schools Regulations now require compliance by schools; giving children and young people a healthy balance of food and drink throughout the entire school day.
“Statutory guidance on the Healthy Eating in Schools Regulations is currently being prepared and will be issued in the new year.”